Pure spinel cobalt oxide nanoparticles were prepared through hydrothermal approach using different counter ions. First, the pure and uniform cobalt carbonate (with particle size of 21.8-29.8 nm) were prepared in high yield (94%) in an autoclave in absence unfriendly organic surfactants or solvents by adjusting different experimental parameters such as: pH, reaction time, temperature, counter ions, and (Co2+:CO32-) molar ratios. Thence, the spinel Co3O4 (with mean particle size of 30.5-47.35 nm) was produced by thermal decomposition of cobalt carbonate in air at 500 degrees C for 3 h. The products were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), transmission electron microscope (TEM), scanning electron microscope (SEM), and thermal analysis (TA). Also, the optical characteristics of the as-prepared Co3O4 nanoparticles revealed the presence of two band gaps (1.45-1.47, and 1.83-1.93 eV). Additionally, adsorption of methylene blue dye on Co3O4 nanoparticles was investigated and the uptake% was found to be >99% in 24 h. (c) 2012 Elsevier Ltd. All rights reserved.